At the local level here, I made a stab at using
quarter round and butt joints to get an edge treatment.
The quarter round a.) was not milled to the same dimension
as the plywood I have and b.) trying to cut oak quarter
round in a $3 miter box is not real reliable.

So I'm wondering what can be suggested here for joinery besides
butt joints and biscuits. Splines have occured to me, but ripping
the edge of plywood is asking for trouble.

I got curious about what could be found at a picture
frame shop. But they prob'ly charge for that stuff by the
inch.